In general, in a hydraulic circuit provided in a hydraulic working machine such as a hydraulic shovel, an open center control system is widely known in the related art. An open center control system is equipped with a variable displacement hydraulic pump, an open center type flow rate control valve configured to control a supply flow rate to a hydraulic actuator from the hydraulic pump, a center bypass oil passage extending from the hydraulic pump to the oil tank via a bleed-off opening of the flow rate control valve. A negative control throttle is disposed on the center bypass oil passage on the downstream side of the bleed-off opening to generate a negative control pressure, and is configured to perform increase and decrease control of the pump flow rate of the hydraulic pump in accordance with the negative control pressure.
However, in the open center control system configured to control the pump flow rate by the negative control pressure using the open center type flow rate control valve, there was a problem in that energy saving was impeded by the large bleed-off flow rate flowing from the center bypass oil passage to the oil tank.
Therefore, a technique of controlling the pump flow rate according to a virtual negative control pressure calculated by a controller has been proposed in Japanese Unexamined Patent Application Publication No. 2013-148174. The bleed-off flow rate is reduced by reducing the bleed-off opening area formed in the flow rate control valve while providing no negative control throttle in the center bypass oil passage.
However, the system disclosed in the '174 publication is configured to perform a flow control of the hydraulic pump by calculating a virtual negative control pressure based on a manipulation amount of a hydraulic actuator manipulation tool (such as an operator lever or pedal) and a discharge pressure of the hydraulic pump, and by outputting the virtual negative control pressure using an electromagnetic proportional valve. Therefore, when there is a failure of a pressure sensor configured to detect the pump discharge pressure or the electromagnetic proportional valve, there is a problem that it is not possible to control the pump flow rate and operate the hydraulic actuator. Further, the flow rate control valve used in the '174 publication is a special flow rate control valve having a small bleed-off opening area. If an existing system using the conventional open center type flow rate control valve is changed to the published control system in order to reduce the bleed-off flow rate, there is a problem of increase in cost since special flow rate control valves at least equal in number to the hydraulic actuators need to be prepared.
Additionally, the bleed-off opening area of the flow rate control valve in the '174 publication is set to be considerably smaller than the bleed-off opening area of a conventional open center type flow rate control valve. Thus, it is expected that the bleed-off flow rate decreases and damping of the hydraulic system gets worse. Also, since the virtual negative control pressure and the virtual bleed-off flow rate are calculated using the pump discharge pressure detected by a pressure sensor, there is also a problem that hunting is likely to occur. Furthermore, the bleed-off opening area is considerably narrowed when the manipulation amount of the hydraulic actuator manipulation tool is small, that is, when the hydraulic actuator supply opening of the flow rate control valve is open only a little. Accordingly, when the manipulation tool is operated in phases or when a reversing operation of suddenly switching the manipulation tool in an opposite direction is performed, because the bleed-off opening area to bleed off the pump flow is too narrow, there is a problem that the pump pressure rapidly increases and operability deteriorates. The present invention seeks to solve one or more of these problems.